The Drosophila polo and Saccharomyces cerevisiae CDC5 genes encode serine/threonine kinases which are highly related in sequence (47% identity in the catalytic domain) and in function (Llamazares, S. et al., (1991) Genes Dev. 5,2153-2165, and Kitada, K. et al, (1993) Mol. Cell. Biol. 13, 4445-4457). Phenotypic analysis of mutant alleles of polo and CDC5 indicates that both are required for the proper formation and function of the spindle during mitotic and meiotic cell divisions (Llamazares, S. et al., (1991) Genes Dev. 5, 2153-2165, 1991; Fenton, B. & Glover, D. M. (1993) Nature 363, 637-640; Sharon, G. & Simchen, G. (1990) Genet. 125, 475-485; Schild, D. & Byers, B. (1980) Genet. 96, 859-876). Two related murine genes, snk (Simmons, D. L., Neel, B. G., Stevens, R., Evett, G. & Erikson, R. L. (1992) Mol. and Cell. Biol. 12, 4164-4169) and plk (Clay, F. J., McEwen, S. J., Bertoncello, I., Wilks, A. F. & Dunn, A. R. (1993) Proc. Natl. Acad. Sci. USA 90, 4882-4886) have recently been cloned and may also play a role in regulating cell proliferation based on cell cycle dependent changes in their transcription levels and a correlation between the tissue specific pattern of plk expression and active cell division.